1. Field of the Invention
The invention relates to a method of manufacturing a front-end optical component in which a lens is mounted on the front end of the lens-barrel provided at the front-end of the inserting portion of an endoscope.
2. Description of the Related Art
Although a front-end optical component configuring the inserting portion of an endoscope has a lens mounted at the front end of the lens-barrel, the lens is bonded and fixed to the inside surface of the lens-barrel with a epoxy-based adhesive. For example, in the front-end optical components of the endoscopes disclosed in the gazettes of Japanese Patent Laid-Open No. 10-234652 and Japanese Patent Laid-Open No. 10-170794, both of the lens are fixed to the lens-barrels with adhesives.
The main body of the front-end portion in the inserting portion of an endoscope is provided with observing means having an objective optical system positioned in the proximity of illuminating means. The objective optical system is usually configured by a plurality of lenses, and these lenses are mounted in a lens-barrel. Herein, a first lens located on the nearest side to a body to be observed of the lens-barrel is a planoconcave lens, of which concave surface is adapted to be located within the lens-barrel. For example, during testing, a pollution substance, such as a body fluid, can adhere to the surface of the first lens exposed to the outside at the front end of the inserting portion, and the adherence of the pollution substance may impair the visual field of observation. Therefore, the inserting portion is provided with a lens surface-cleaning mechanism for washing out pollution substances from the surface of the first lens. The lens surface-cleaning mechanism is provided with a nozzle for issuing a jet of cleaning fluid toward the outside surface of the first lens, and issues a jet of cleaning fluid, usually washing water, from this nozzle to wash out pollution substances. Then, the mechanism blows pressurized air on the lens surface to remove washing water remaining on the lens surface. Further, when existing in a body cavity, the first lens is in a state of about the same temperature as the body temperature. In the case of an electronic endoscope, the first lens can be in a state of a higher temperature than the body temperature due to the existence of a heating element, such as a solid image sensor. On the other hand, the washing water is not particularly heated up, and therefore the temperature of the washing water is held about at the same temperature as the room temperature. For this reason, when the washing water is jetted on the outside surface of the first lens, the first lens can be quickly cooled. As a result of cooling the first lens, if moisture is contained in the air in the lens-barrel, fogging or condensation can be caused on the inside surface side of the first lens. Moreover, because the inside surface of the first lens is configured by a concavely curved surface and its curvature is large, the temperature decrease is most remarkable at the central portion thinnest in thickness and its neighborhood in the first lens, thus resulting in fogging or condensation concentrated on the central portion in the concave surface of the first lens. Since light beams necessary for image formation is concentrated in the central portion of the concave surface of the first lens, least fogging of this region would cause rapid decrease in the image quality of observational images obtained, thereby resulting in a very difficult-to-look image. Further, in some other cases, when the front end of the inserting portion is also quickly cooled, there is also a fear that fogging or condensation may take place on the first lens and the like.
In this manner, since fogging or condensation can possibly take place on the lens (first lens) of the front-end optical component of an endoscope, as a measure for preventing this, no moisture-containing gas such as dry air and nitrogen gas has been filled in the space in the lens-barrel.
In the first place, taking in moisture into the lens-barrel is caused by that the first lens is bonded to the lens-barrel with an adhesive and moisture can pass through this adhesive. The adhesive is pervious to water in nature, and therefore moisture enters into the lens-barrel through an adhesive layer between the first lens and the lens-barrel, resulting in fogging of the first lens.